drm/amd/display: fix invalid function table override

Otherwise we try to program hardware with the wrong watermark functions
when multiple DCE generations are installed in one system.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org